Slow-wave circuit for a traveling wave tube



Oct. 28, 1958 KARP 2,858,472

SLOW-WAVE CIRCUIT FOR A TRAVELING WAVE TUBE I Filed Oct. 16, 1953 2Sheets-Sheet 1 lNl/EN r09 A. KARP ATTORNEY Oct. 28, 1958 A. KARP2,858,472

SLOW-WAVE CIRCUIT FOR A TRAVELING WAVE TUBE.

Filed Oct. 16, 1953 2 Sheets-Sheet 2 MODE WA l/E lNVENTOR ,4. KA/PPATTORNEY tates Fatented ct. 28, 1958 Laboratories, Incorporated, NewYork, N. Y., a corporation of New York Application October 16, 1953,Serial No. 336,582

14 Claims. (Cl. 315-3.6)

This invention relates to electromagnetic wave phase retarding circuitsand more particularly to slow wave circuits suitable for use intraveling. wave tubes.

An object of this invention is to provide. an improved wave propagationcircuit especially suitable for use in high power millimeter wavelengthtraveling wave tubes. A more specific object is to provide such acircuit which is adapted for use with circular transverse electric modeWaves.

The invention of the traveling wave tube was the first step in thedevelopment of many of the present day techniques in the generation andamplification of millimeter wavelength electromagnetic waves. Althoughthe common type of traveling wave tube employing a wave propagatinghelix was limited by requirements of physical size to operation atwavelengths generally longer than one centimeter, recent discoveries,such asth'efspatial harmonic traveling wave tube disclosed in U. 8.Patent 2,683,238, issued July 6; 1954, to S.- Millman, have extendedthis rangeto less than one-half centimeter.

These advances in the art have aroused interest in the possibility oftranscontinental transmission of millimeter waves via closed waveguides. A single wave guide can accommodate signal frequencies over arange of many thousands of megacycl es and could theoretically replacemost if not all existing transcontinental lines. Serious dimc'ulties inlong distance wave guide transmission, howeven'remain to be overcomebefore such transmission is practical. Among these, perhaps one of themore prominent is the, lack of broad-hand high-power tubes which arerugged and easy to manufacture. The present invention is intended tosupply such a tube.

This invention is based upon the spatial harmonic principle of waveamplification disclosed in the above mentioned patent. Brieflyexplained, spatial harmonic amplification takes place when a stream ofelectrons is beamed with the proper velocity in coupling proximity witha Wave propagating circuit of the iterative filter type having anon-zero distance between sections. Neglecting losses, such a circuit,.by virtue of its periodic nature, causes themaximum, as distinguishedfrom the instantaneous, amplitude of the electric field to be a periodicfunction of distance along the circuit. Thus at a given instant of timethe peak electric intensities along the circuit are not all the" samevalue, contrary to the condition along a uniform structure, such as alossless wave propagating coaxial cable, where the .peakintensities. areequal. As a result of this nonuniformity, the over-all electric fieldappears to be composed of a doubly infinite series of spatial harmoniccomponents each having the same frequency but each having a phasevelocity of propagation diiferent from that of any other compermit inthe series; By matching the velocity ofthe electron stream with thephase velocity of one of these components, it' is possible to extractkinetic energy from the electrons, which is a condition for waveamplification, even though the phase velocity of the fundamentalcomponent of the electromagnetic wave is much greater than the velocityof the electron stream.

In accordance with the present invention, a number of spatial harmoniccircuits are disposed circularly within a conductively bounded waveguiding passage in such a way that they can be used in conjunction withan electromagnetic wave propagating within the guide in the fundamentaltransverse electric circular mode. (This mode is commonly designated TEand its field configuration is shown in Fields and Waves in Modern Radioby Ramo and Whinnery, John Wiley, 1946, pages 338 and 339.) One or moreelectron streams can then be beamed in coupling relation to thesecircuits in order to produce wave amplification. Among the advantages ofsuch an arrangement are structural ruggedness, increased power handlingcapacity, and ease of manufacture.

A more complete understanding of the general nature of the invention,together with a better appreciation of its numerous advantages, willbest be gained from a study of the following detailed description givenin connection with the accompanying drawings in which:

Fig. 1 is a perspective view of a first illustrative em ,bodiment of theinvention;

Fig- 2 is a cross section view of the embodiment of Fig. l;-

Flg. 3 is a fragmentary cross section view of a second iilustrativeembodiment of the invention;

Fig. 4 shows a modification of the embodiment of Fig. 3; and

Fig. 5 illustrates in longitudinal cross section an arrangement forbeaming electrons past the embodiments of Fig. 1 of the invention and,in addition, shows one way of impedance matching into and out of thisembodiment.

Referring now in detail to the drawings, Fig. 1 shows by way ofillustration of the present invention a slow wave structure 10comprising a number of spatial harmonic circuits 11 arranged circularlyabout the axi's of a conduc'tively bounded wave guiding passage ofradius r This passage, together with the spatial harmonic circuits 11,is in this embodiment formed by a plurality of metal stampings or plates12 having a thickness t and spaced a distance d between opposingsurfaces. Plates limay be formed in any convenient way such as byphotographic etching of a suitable conductor such as copper or goldplated molybdenum. The openings in them are made so that crescent shapedWedges or segments 13 are positioned in the region where, in the absenceof the wedges, there would exist the maximum field intensity of a wavepropagating in the fundamental circular transverse electric mode.Segments 13 are supported in place by struts 14 which connect them tothe main body of the plates. Gaps 15 each having an angle 0 betweensegments form an essential part of circuits 1i and it is in and aroundthese gaps that the electric field of a wave propagating down structure10' can have a relatively strong electric field component parallel tothe direction of wave propagation. It is this component which is able tointeract with one or more electron streams beamed in these regions. Anysuitable means (not shown) may be used for generating the electronstreams and for focusing them along paths in the vicinity of gaps 15. Asuitable means is shown in the abovementioned Millman application.

It should be noted that, while spatial harmonic circuits 11 are formed,in the embodiment shown in Fig. l, by groupsof segments 13 supportedaround openings in longitudinally spaced plates 12, these circuits maybe formed by longitudinally spacing similar groups of segments of thesame general configuration along the inside of a circular conductingwave guide. The spaced-plate 3 arrangement of Fig. 1, however, helps tomaintain the purity of the TE mode by suppressing unwanted modes. Ineither structure though the number of longitudinally spaced segmentsused depends .upon the amplification or gain required in the travelingwave tube utilizing them.

At each end of the embodiment of Fig. 1 some form of impedance matchingto and'from a cylindrical wave guide should generally be used. The wayshown at the right end of structure consists of tapering the area ofsegments 13 gradually from full size down to roughly zero size over adistance several guide wavelengths long. It should be understood,however, that this is not the only possible arrangement since othersequally effective may be used instead. The left end of structure 10 hasbeen shown for simplicity in Fig. 1 without any form of impedancematching.

When slow wave structure 10 is operated as a traveling wave tube, one ormore electron streams can be beamed within an evacuated envelopelengthwise through the spaces provided by gaps so that the electronsinteract with an electromagnetic wave of'proper configuration applied tothe structure by such means as a hollow circular wave guide. The phasevelocities of propagation of the spatial harmonic components of thiswave along circuits 11 are determined principally by the physicaldimensions of segments 13, gaps 15, distance d, and thickness t,

and these velocities can be computed if desired by the method, now wellknown to the art, described in the above-mentioned patent.Alternatively, these velocities may be determined from the plot of thephase propagation constant 5 measured as afunction of the frequency ofthe wave applied to circuits 11. As explained previously by matching oneof these velocities with the velocity of an electron stream, energy canbe transferred from the electronsto the traveling electromagnetic wave.

Electrons traveling longitudinally down circuits 11 in or near gaps 15see substantially no longitudinal field as they pass directly over theconductive surfaces of segments 13 (since there can be no electric fieldtangential to a metal surface) while when passing between plates 12 theysee a strong longitudinal electric field. This alternate passage fromdrift space to interaction space is analogous to a stroboscopic lightflashing on a patterned velocity of the wheel corresponding to the phasevelocity of the fundamental spatial harmonic component of the travelingwave. For a given wheel velocity there will be several discretestroboscopic frequencies at which the wheel appears stationary and eachof these apparent nonrotations of the wheel corresponds to synchronismbetween a spatial harmonic component of the Wave propagating alongcircuits 11 and the electrons. In this synchronous condition a singleelectron sees the same electric field vector as it passes through eachregion between plates 12. Thus the requirement for electromagneticwaveelectron-stream interaction is met by, in effect, fooling the electrons.

By assuming that the group velocity of the wave propagating down thestructure is opposite to the velocity of electron flow, it can be seen,following the above analogy, that the electrons can be synchronized withthe spatial harmonic component of the wave having a negative phasevelocity relative to the group velocity. When such conditions actuallyexist in a spatial harmonic tube, electromagnetic power flows from thecollector end to the gun end of the tube. This mode of operation, usefulfor amplification up to a critical value of beam current, is likewiseuseful for obtaining oscillations beyond this critical value since thenecessary feedback path for sustaining the oscillations is thenautomatically provided by the electron stream.

Fig. 2 is a cross sectional view of structure 10. Members 13 and 14 aredisposed symmetrically around the circular opening of radius r in plate12. The number of segments 13 in each plate group depends upon a numberof factors such as the power handling capacity desired or the over-alldiameter (2r required. In general, it is probably desirable to use aprime number, such as the five shown, in order to minimize the danger ofindividual circuits working together in sub-groups and producing outputsat the end of the tube that differ in phase or frequency. The physicaland electrical relations of these circuits to each other in thearrangement shown, however, are by themselves important factors inreducing this danger since all parts of the circuit are tied together byvirtue of the existence of a single mode Wave.

Area 21 in Fig. 2 indicates the cross section and position of anelectron stream which can be beamed through and around gaps 15. Areas22, 23, and 24 indicate the cross sections and positions of alternativeelectron streams which can be accommodated by properly slotting oraperturing wedges 13 as shown here but not shown in Fig. 1.

In an experimental model substantially the same as that I illustrated inFigs. 1 and 2, in which r =l inch, r r

and 0 bear the same relation to r as shown in Fig. 2, t=0.075 inch andd=0.034 inch, an electron stream accelerating voltage of approximately1200 volts was found to be sufficient to synchronize the electrons witha 9000 megacycle wave propagating down the structure.

Fig. 3 is a second illustrative embodiment of the invention shown inpartial cross section in which a slow wave structure 30, substantiallythe same in principles of construction and in operation as theembodiment in Fig. l, is formed by a plurality of wire loops 31 and 32supported symmetrically by struts 34 and 35, respectively, aroundcircular openings of radius r in plates 33. Plates 33 are spacedlongitudinally in the same way as plates 12 are spaced in Fig. 1. Inthis embodiment, however, every other plate 33 is rotated angularly (pdegrees with respect to the adjacent plates so that groups of loops 31in one plate and groups of loops 32 in a different plate will bedisplaced relative to each other as shown, thereby forming spatialharmonic circuits of the interdigital type. In such circuits the phaseshift of the electric field between successive discontinuities, such asbetween a loop 31 and a loop 32 adjacent thereto, is approximately 1rradians over a considerable frequency range and this constant phaseshift is the cause of the wide operating bandwidth of a backward modeinterdigital type traveling wave tube. When the structure of Fig. 2 isused as an amplifier or oscillator one or more electron streams may bebeamedthrough the dotted areas 36.

Fig. 4 shows a variation of the structure shown in Fig. 3. Here a groupof posts or fingers 41 lying in the plane of one plate 42 is rotatedangularly with respect to a group 43 lying in the plane of an adjacentplate 42. Electrons may be beamed through areas 44. Impedance matchingcan most easily be accomplished by tapering the lengths of post 41instead of their widths as was done with segments 13in Fig. 1. Thisarrangement has the advantage of structural simplicity.

Fig. 5 shows a portion of the longitudinal cross section taken asindicated by lines 5--5 in Fig. 1, of a traveling wave tube embodyingslow wave structure 10. One or more electron guns 51 located around theoutside of circular wave guide 52 beam' electrons lengthwise down thepaths indicated by area 21 in Fig. 2. Collector electrodes (not shown)may be similarly located at the other end of the tube. An alternative tothe use of guns 51 lies in the use of cathodes coated directly upon thefirst few of segments 13. A tapered length of guide 53 extending over alength which can be roughly two guide wavelengths long serves to matchthe impedance of guide 52 aims to" that of structure A similarTtaperedsection may be" usedlat the opposite and although forconvenience this has not been shown.

Structure 10 isarranged so. that electron beam focus ing can bea'ccompl'ished in any one of several different Ways in addition tofocusing. by the customary uniform longitudinal magnetic field. Focusingis caused here by the periodic direct voltage electric field existing inthe region between ,plates l l. The optimum intensity of this field formost etficient focusing can be easily found by varying voltage source54' which is connected between plates 12 in the way shown. Periodicmagnetic focusing can be achieved in an analogous Way by insertingpermanent magnets in the openings between plates 12.

When tube '50 is operating as a forward mode spatial harmonic amplifier,wave energy in the proper mode which may be obtained directly from acircular wave guide propagating a TE f wave or through appropriatetransducers, from other guides isapplied to structure 10 via guide 52'.The amplified signal may then be extracted at the other end of structure10 by any appropriate means. When operating in a backward wave mode,wave energy is extracted from tube 50 via guide 52.

While the foregoing will serve to illustrate the pertinent details ofthe present invention, it is not intended as a complete exposition ofall possible embodiments which can be devised according to theprinciples set forth. Various modifications and changes in the geometryor physical relations of structure 10 or 30 and in the ways of utilizingthese structures will occur to those skilled in the art and may be madewithout departing from the spirit or scope of the invention.

What is claimed is:

1. In combination, means forming a path of electron flow, a waveinteraction circuit for propagating electromagnetic wave energy incoupling proximity with said electron flow, said wave interactioncircuit comprising a plurality of conductive members arranged in spacedsuccession along said path of flow and insulated from each other, eachconductive member being substantially transverse to said electron pathand including a plurality of conductive segments circumferentialy spacedaround said electron path.

2. The combination of elements as in claim 1 in which the conductivesegments are sectors of a solid metallic annulus.

3. The combination of elements as in claim 1 in which the conductivesegments are metallic wire loops.

4. The combination of elements as in claim 1 in which the conductivesegments are radially extending metallic posts.

5. In combination, means forming a path of electron fiow, a waveinteraction circuit for propagating electromagnetic wave energy incoupling proximity with said electron flow, said wave interactioncircuit comprising a plurality of conductive members arranged in spacedsuccession along said path of flow, each conductive member beingsubstantially transverse to said electron path and including a pluralityof conductive segments circumferentially spaced around said electronpath, and focusing means including a voltage source for maintainingalternate conductive members of the succession at the same potential andadjacent conductive members at a different potential.

6. In a device which utilizes the interaction between an electron beamand an electromagnetic Wave, means defining a path of electron flow, aninteraction circuit for propagating electromagnetic wave energy incoupling proximity with said electron flow, said interaction circuitcomprising a spaced succession of conductive members along the electronpath, each conductive member being transverse to said electron path andincluding a plurality of substantially uniform conductive segmentscircumferentially spaced around said path, a coupling connectioncomprising a circular hollow wave guide for propa gating energy in thecircular electric mode, and a wave guiding transition section interposedbetween said wave circuit and said hollow wave guide for propagatingelectromagnetic energy therebetween, said transition section comprisinga spaced succession of conductive members, each conductive membercomprising a plurality of circumferentially disposed conductivesegments, the dimensions of said segments increasing along the wavetransition section in the direction from the hollow Wave guide to theinteraction circuit.

7. In combination, an interaction circuit comprising wave guding meansfor propagating electromagnetic wave energy along an axis substantiallyin the fundamental transverse circular electric mode, said meanscomprising a spaced succession of conductive members insulated from oneanother along said axis each of said members having a plurality ofradial slots therein, and means for generating an electron beam andprojecting said beam through said radial slots.

8. In combination, a coupling connection comprising a circular hollowwave guide for propagating an electromagnetic wave along an axis in thefundamental transverse circular electric mode, a wave guiding transitionsection coupled to said coupling section comprising radially extendingconductive means for confining the electromagnetic wave energy intosectoral regions, said radially extending conductive means comprising asuccession of groups of radially extending conductive elements, eachgroup lying in a plane transverse to said axis and spaced apart fromadjacent groups of said succession in a direction parallel to said axis,the angle subtended by said radially extending members increasing alongthe length of said transition section, interaction circuit means coupledto said transition section, said interaction circuit comprising asuccession of groups of radially-extending substantially uniformconductive elements, each group lying in a plane transverse to said axisand spaced apart from adjacent groups of said succession in a directionparallel to said axis, and means for projecting a beam of electronsparallel to said axis for interaction with the wave energy passing alongsaid interaction circuit means.

9. A traveling Wave tube amplifier for amplifying energy in thefundamental transverse circular electric mode comprising electrodesspaced apart for defining therebetween a path of electron flow, Waveinteraction means positioned along said path of electron flow forpropagating electromagnetic wave energy in coupling proximity with saidelectron flow, said interaction circuit comprising a succession ofgroups of conductive members along an axis parallel to said path offlow, each group lying in a plane transverse to the path of electronflow and spaced apart from adjacent groups of said succession along saidaxis, and the conductive members of each group being uniform andcircumferentially spaced about said axis, a circular hollow wave guidecoupling connection to said traveling wave tube amplifier, and impedancematching means for coupling wave energy between said circular hollowwave guide and the interaction circuit, said impedance matching meanscomprising a succession of groups of conductive members along the axisof the circular hollow wave guide, each group lying in a planetransverse to said wave guide axis and spaced apart therealong, theconductive members of each group being circumferentially spaced aboutsaid wave guide axis, and the angle subtended by the conductive membersincreasing along said succession away from the circular hollow waveguide.

10. in an interaction device for amplifying energy in the fundamentaltransverse circular electric mode a section of circular hollow waveguide, an impedance matching section axially aligned with said sectionof circular hollow wave guide for receiving wave energy therefrom, aninteraction circuit positioned along the same axis and forming acontinuation of the impedance matching section for receiving wave energytherefrom, and means for defining a path of electron flow along thelength of the interaction circuit and substantially parallel to saidaxis in coupling proximity to the Wave energy propagating along theinteraction circuit, said interaction circuit comprising a succession ofgroups of conductive elements, the conductive elements of each groupcircumferentially arranged about the axis and successive groups spacedapart along the axis, and the impedance matching section comprising alike succession of groups of conductive elements wherein the anglesubtended by the circumferentially arranged conductive elementsincreases along the axis of the impedance matching section from thecircular hollow Wave guide section to the interaction circuit.

11. In combination, interaction circuit means for propagating along anaxis an electromagnetic wave substantially in the fundamental transversecircular electric mode, said means comprising a linear array of groupsof conductive elements, each group lying in a plane transverse to saidaxis and successive groups spaced apart along the axis, impedancematching means positioned along the axis of propagation of saidfundamental transverse circular electric mode, and means for projectingan electron beam in coupling proximity to said interaction circuit andsubstantially parallel to said axis.

References Cited in the file of this patent UNITED STATES PATENTS2,547,503 Smith Apr. 3, 1951 2,640,951 Kuper June 2, 1953 2,643,353Dewey June 23, 1953 2,645,737 Field July 14, 1953 2,653,270 KompfnerSept. 22, 1953 2,683,238 Millman July 6, 1954

